Level II scour analysis for Bridge 23 (HARDTH00530023) on Town Highway 53, crossing Haynesville Brook, Hardwick, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
HARDTH00530023 on Town Highway 53 crossing Haynesville Brook, Hardwick,
Vermont (figures 1–8). A Level II study is a basic engineering analysis of the site, including
a quantitative analysis of stream stability and scour (U.S. Department of Transportation,
1993). Results of a Level I scour investigation also are included in Appendix E of this
report. A Level I investigation provides a qualitative geomorphic characterization of the
study site. Information on the bridge, gleaned from Vermont Agency of Transportation
(VTAOT) files, was compiled prior to conducting Level I and Level II analyses and is
found in Appendix D.
The site is in the New England Upland section of the New England physiographic province
in north-central Vermont. The 14.2-mi²
drainage area is in a predominantly rural and
forested basin. In the vicinity of the study site, the predominate surface cover consists of
field grasses except for the upstream left bank with is brush covered.
In the study area, Haynesville Brook has a sinuous channel with a slope of approximately
0.004 ft/ft, an average channel top width of 39 ft and an average channel depth of 2 ft.
Stream-bed material at the site ranged from silt to gravel with a median grain size (D50) of
49.9 mm (0.164ft). The geomorphic assessment at the time of the Level I and Level II site
visit on July 27, 1995, indicated that the reach was laterally unstable. Channel scour in both
the upstream and downstream reaches as well as irregular point bars and cut banks and
upstream anabranching led to this assessment.
The Town Highway 53 crossing of Haynesville Brook is a 33-ft-long, one-lane bridge
consisting of one 26-foot steel-beam span (Vermont Agency of Transportation, written
communication, March 24, 1994). The bridge is supported by vertical, concrete abutments
with no wingwalls. The concrete may be facing over the original stone abutments. Sheet
piling has been driven around the base of each abutment and is filled with concrete. The
channel is skewed approximately 10 degrees to the opening; the opening-skew-to-roadway
is also 10 degrees. Additional details describing conditions at the site are included in the
Level II Summary and Appendices D and E.
Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a
highway crossing is comprised of three components: 1) long-term streambed degradation;
2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge)
and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is
the sum of the three components. Equations are available to compute depths for contraction
and local scour and a summary of the results of these computations follows.
Contraction scour for all modelled flows ranged from 0.0 to 2.0 ft. The worst-case
contraction scour occurred at the 100-year discharge. Abutment scour ranged from 7.0 to
12.9 ft. The worst-case abutment scour occurred at the 500-year discharge. Additional
information on scour depths and depths to armoring are included in the section titled “Scour
Results”. Scoured-streambed elevations, based on the calculated scour depths, are presented
in tables 1 and 2. A cross-section of the scour computed at the bridge is presented in figure
8. Scour depths were calculated assuming an infinite depth of erosive material and a
homogeneous particle-size distribution.
It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.